It has long been the accepted wisdom that the magnificent temples at Angkor in Cambodia — with Angkor Wat the most famous — were only the tip of the proverbial iceberg of what was once the Khmer Kingdom. There is almost no written record from the period when the Khmer dominated much of Southeast Asia and its kings built hundreds of massive stone structures. Many of those structures have been found, along with inscriptions that reflect critical information about Khmer royalty, but little information has been available about Khmer society as a whole. Until recently, all we have had to go on are accounts from travelers, circumstantial evidence, and a relatively small number of artifacts that point to the previous existence of major cities and additional temples in what is now Cambodian jungle.

That’s all changed thanks to some impressive and extensive use of lidar-equipped helicopters to accurately chart the ground underneath the dense foliage. To an impressive extent, the lidar data shows where walls, and other large structures like moats, canals, roads, ponds, quarries, forges, and smelters, were located. Previously, it was necessary to physically clear and excavate sites before knowing whether there was anything to find. Now, with the lidar data, archaeologists can precisely target their efforts.

How lidar measures the ground

The idea behind lidar (short for light detection and ranging) is simple. A laser is projected and its return is precisely timed, allowing the device to closely-measure the distance of whatever reflected the lidar. If the laser is then scanned across an area, an accurate depth map of the surroundings can be created. Most of us are familiar with the use of lidar for self-driving cars and obstacle-avoiding drones, but it is also excellent for building models of building and terrain. To accomplish that, the laser emitter and detector have to be coupled with a high-precision clock and a GPS and IMU so that its output can be correctly placed on a map.

A simple, or single-return, lidar only measures the distance to the first object encountered. But more-sophisticated, multiple-return, lidar systems can measure up to four return values for each laser pulse (along with an intensity value for each of those). The first return will typically be the top of whatever foliage is located where the pulse hits, and subsequent returns would either be lower portions of foliage or, eventually, the ground itself. The laser doesn’t pass through trees or leaves, but there are typically enough gaps that some of the pulses will make it to the ground and be measured. Post-processing the data allows for maps of both the foliage and the terrain to be made from the same set of scans.

How lidar makes the invisible, visible

It’s not intuitive that accurately measuring ground elevations will highlight where walls were 1,000 years ago. In fact, you can walk over many of those areas and not notice the variation in height with your eye. In this case, most of the ground is also covered with dense forest, making visual observation and measurements impossible. Lidar, though, solves both of these problems, by being extremely accurate and by cutting through the foliage.

This illustration shows clearly how much data is available a lidar-scan — shown on the right — compared to a traditional photographs. Credit: “Damian Evans / Journal of Archaeological Science”

Building on earlier work

The recent, expanded, lidar survey builds on an initial effort undertaken under the guidance of a team of researchers led by the University of Sydney’s Dr. Damian Evans and Professor Roland Fletcher. That survey of 370 square kilometers undertaken in 2012 helped settle once and for all the debate over whether the Khmer civilization was a disconnected set of small cities and towns or one massive-and-interconnected urban sprawl — one that relied on an extremely-large set of irrigation projects for its high crop yields. The study lent additional credence to the theory that a combination of factors, including a change in the climate, caused those yields to falter and the civilization to decay. It also documented the sheer scale of the cities. The largest, built around the Angkor Thom “city temple” was previously estimated to include the 9 square kilometers enclosed by the temple walls, but was shown in the scans to encompass 35 square kilometers.

Reservoirs like this one — Srah Srang — have been proven to play a key role in the success of the Khmer agriculture, and thus the Khmer civilization for several centuries.

The most recent work, a much larger 734-square-mile scan, has helped address the next piece of the Khmer puzzle. Many scientists had postulated that the Khmer abandoned their cities near Angkor and fled when crops began to fail. But the new scans, and other scans undertaken as part of other projects, have shown that there weren’t any alternate city sites, so the residents clearly didn’t resettle elsewhere en masse. The teams full report will be published in the Journal of Archaeological Science later this month.

Mahendraparvata: Discovering an entire city using lidar

It has long been speculated that there was a city that pre-dated Angkor in the area of Phnom Kulen — the site where the first Khmer King, Jayavarman II, declared independence from Java. Everyone, especially including the team behind the recent lidar overflights, expected it to be there. It had a name, Mahendraparvata, but not much beyond circumstantial evidence that it actually existed. That changed with the first round of scanning in 2012, which allowed the full scope of the ancient city to unfold in detail on the computer screens of Damian Evans and his team. Perhaps inspired by the dramatic revelations of the 2012 work, the team got funding for the much-more-extensive 2015 project.

A boon to tourism?

Since being reopened to tourism two decades ago, Cambodian temples have seen explosive growth in the number of visitors nearly every year — from a few hundred thousand the first year I visited in 2005, to millions annually now. Perhaps fortunately, both the difficulty of finding and restoring them in the jungle, and the large amount of un-exploded ordinance near many of them kept them off the tourist track until they could be properly protected and carefully restored.

Landmines and poor roads kept Beng Melea, off the tourist route for many years. Unlike Angkor, it has been left mostly in the same semi-ruined state in which it was found. This “library” is only a small portion of the forest-covered expanse.

Until recently, Cambodian temples have been a victim of their own success. The main temples have gotten very crowded, and many who had Angkor Wat on their bucket list have now “seen it” and checked it off as part of a larger tour. Now, however, it’s clear that there is a lot more history to be discovered and explored, possibly in more of an ecotourist-inspired way. Even if more tourists don’t come, the additional finds will provide a better experience for those who do, as they will be able to spread out across even more sites.

Images like this one of the classic tree growing through Ta Phrom aren’t possible any more. Abuse by careless tourists has forced the construction of a viewing platform and railing.

While the Angkor lidar projects are groundbreaking in many ways, they are definitely far from the first. Lidar has also been used to investigate the extent of Mayan ruins, for example. Longer-term, I think this is only the beginning of a new phase of exploration. Combining lidar, multi-spectral imaging, and AI for recognizing patterns is likely to provide an exciting set of new discoveries in Cambodia and elsewhere in the world where ancient secrets lay hidden.